We propose a collaborative project between Texas A and M University and the Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, in Moscow in the area of the conformational stability of enzymes. Specifically, we plan to test the hypothesis that protein residues contributing to catalysis are not optimal for protein stability, using microbial ribonucleases as models. We will study the effect of changes in the active site residues that abolish or diminish the enzyme's catalytic activity on the enzyme's thermostability using the RNases from Bacillus intermedius, binase, and Streptomyces aureofaciens, RNase Sa. In addition, we will estimate the stabilizing effect of restricting the mobility of particular residues in the RNase active sties by assessing the melting of proteins in complexes with the protein inhibitor barstar and its mutants. The primary goal of the investigation is to ascertain the existence in RNases of an interrelation between the thermostability of the protein and the conformational flexibility of active site residues required for catalytic efficiency. The Moscow group has recently observed an inverse dependence of enzymatic activity on thermal stability for chimeric RNases composed of different parts of barnase and binase molecules. The significance of the proposed project is that we will learn to predict how substitutions at active site residues will affect the function, folding, and stability of an enzyme. This can have future potential for redesign of enzymes to generate the desired catalytic properties and stability.